Grain oriented silicon steel sheet having low core loss and method of manufacturing same

A method of forming an insulating coating on a grain oriented silicon steel sheet comprising:

• providing a final annealed grain oriented silicon steel sheet having a silicon content of from 2 to 4.8 percent by weight and which does not have an inorganic mineral surface layer on a surface thereof;
• forming an external oxidation SiO.sub.2 layer having a film structure and having a layer thickness of not less than 0.001 .mu.m on said surface of said final annealed grain oriented silicon steel sheet not having the inorganic mineral surface layer thereon by subjecting said final annealed grain oriented silicon steel sheet to soaking at a temperature of 700 to 1000.degree. C. in a controlled weakly oxidizing atmosphere wherein P.sub.H20 /P.sub.H2 .ltoreq.0.15, wherein P.sub.H20 is water vapor partial pressure in the atmosphere and P.sub.H2 is hydrogen partial pressure in the atmosphere, said external oxidation SiO.sub.2 layer resulting from silicon in the steel sheet diffusing to the surface of the steel sheet not having the inorganic mineral surface layer and oxidizing on said surface to form said external oxidation SiO.sub.2 layer;
• forming a tension-imparting insulating coating on said external oxidation SiO.sub.2 layer on said surface of said final annealed grain-oriented silicon steel sheet not having an inorganic mineral surface layer thereon.